The present invention relates to an optical printer which writes an information signal representative of an image to be recorded into a photoconductive element by converting the signal to optical information and, more particularly, to an optical information writing dot array fluorescent tube adapted to transform the information signal into optical information.
Optical printers may generally be classified into two types, i.e., a type which raster scans optical dots to project them to a photoconductive element such as a laser type system and an OFT type system, and a type which projects an array of optical dots corresponding to one line onto a photoconductive element such as a light emitting element (LED) type system, a fluorescent tube type system and a liquid crystal type system. The type to which the present invention constitutes an improvement is the second-mentioned dot array type optical printer.
An optical printer of the type with which the present invention concerned includes a dot array fluorescent tube for transforming an information signal into optical information. The fluorescent tube compises a single array of small fluorescent elements each being formed on an anode, and cathode filaments. The arrangement is such that thermoelectrons liberated from the cathode filaments impinge on those fluorescent elements which are selected in response to an information signal, so that the selected fluorescent elements emit light to thereby convert the information signal to optical information. The respect anodes are connected to external drive control devices in order to electrically control their associated fluorescent elements independently of each other and on-off control them by means of information signals (print information signal and others). In this kind of fluorescent tube, a grid electrode is positioned between the cathode filaments and the fluorescent element array so as to effectively conduct the thermoelectrons released from the cathode filaments onto the fluorescent elements. That is, the grid electrode serves as a kind of electronic lens for effectively directing the thermoelectrons toward the fluorescent elements.
In detail, the dot array fluorescent tube usually includes a glass or ceramic substrate on which a plurality of anodes are formed by photoetching, for example, in such a manner as to represent discrete dots such as in a staggered configuration. Each of the anodes carries thereon a fluorescent element which is formed by photoetching, for example, in a size which is 50-60 microns each side in corresponence with a small dot for printing. Usually, the fluorescent elements are made of ZnO:Zn and applied to the respective anodes by electrodeposition or a so-called exposure process which uses photoresist as a binder. The anodes are covered with insulating layers (insulative paste layers) except for their portions where the fluorescent elements are located. A grid electrode is provided on the insulating layers. Having a mesh structure, the grid electrode is so arranged as to cover the anodes carrying the fluorescent elements thereon from above. Two cathode filaments each comprising a thin line coated with an oxide are stretched in a vacuum space above the grid electrode and adapted to liberate the previously mentioned thermoelectrons. The vacuum chamber is hermetically closed by a substrate and a face glass.
The problem encountered with such a prior art dot array fluorescent tube is that since the grid electrode covers the fluorescent element array from above, optical information to be written in a photoconductive element is partly intercepted by the grid electrode, resulting in waste of radiations and irregular radiations.
Meanwhile, it is a prerequisite in the prior art fluorescent tube discussed above that the grid electrode be positioned with accuracy on opposite sides of the minute dot pattern so as to allow the fluorescent elements on the anodes to be stimulated by thermoelectrons from the cathode filaments and, thereby, cause a current to flow through the numerous dots in an even density. In practice, the grid electrode is bonded to a metal sheet by a glass paste having a low melting point. However, whatever the accuracy of the grid position may be, a heating step which is performed after the mounting of the grid electrode for fusing low melting point glass causes the grid electrode to be distorted and, as a result, to be bent or warped. The distortion introduces irregularity locally in the flow of the thermoelectrons and, thereby, irregularity in the luminance of radiation from the dot array.
Further, when light is issuing from the fluorescent elements of the prior art fluorescent tube is to be focused by an imaging element of the optical printer onto the photoconductive element, the light transmitted through the face glass partly misses the imaging element and propagates the outside thereof to provide a flare component. The flare component reaching the photoconductive element has adverse influence on a projected image, thereby lowering the recording quality.
In an optical printer with the prior art dot array fluorescent tube discussed above, assuming that requiring exposure energy on the photocondutive element is 10 erg/cm.sup.2, a printing rate of, for example, ten copies per second has to be implemented by a luminance of light issuing from the fluorescent elements which is as high as 7,000-10,000 fL (on the assumption that the fluorescent elements are made of AnO : Zn and the light utilization efficiency of the imaging element is 5%). This means the need for a radiation capability with a considerable luminance in view of the fact that the luminance ordinarily required for display applications is about 200-1,000 fL. Assuming that each fluorescent element is sized 50 by 50 microns, the luminance of 10,000 causes a current of about 10 microamperes to flow into the element resulting in a substantial current density in the anode. Should the fluorescent elements be used in such a situation, the luminance would be lowered (deteriorated) by aging.
Meanwhile, the anodes of the prior art dot array fluorescent tube are led out to the outside of the face glass in order to be connected with external drive control devices such as integrated circuit (IC) chips. Specifically, it is necessary to control the drive of the anodes by connecting them in one-to-one correspondence with electrodes on an IC substrate which are led out from IC chips. The problem encountered here is that since the pitch of the anode arrangement is small and, in addition, the required length of connection is large, a pitch error is liable to develop between the electrodes on the IC substrate and the anodes on the fluorescent tube substrate. Total pitch error throughout the length of a flexible tape or a printed circuit board, on which the ICs are mounted, a substantial and prevents the connection from being accomplished in a desirable condition. A possible practical approach for the connection is thermocompression bonding. However, where the substrate of the fluorescent tube is made of glass, such an approach is undesirable because it often produces cracks in the substrate or even breaks it during the process. A failure of the fluorescent tube has to be repaired by replacing not only the tube but also the whole, comparatively expensive IC substrate.